The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations and of the kind comprising a bit body having a leading face and a gauge region, a plurality of cutting elements mounted at the leading face of the bit body, and a passage in the bit body communicating with openings in the leading face of the bit body for supplying drilling fluid to said face for cooling and cleaning the cutting elements.
There are many different designs of drill bit of this general type. For example, the bit body may be machined from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in a furnace so as to form a hard matrix. The cutting elements may be mounted directly on the bit body, or may be mounted on carriers, such as studs or posts, which are received in sockets in the bit body.
One common form of cutting element is a preform cutting element having a superhard front cutting face. For example, the cutting element may comprise a hard facing layer of polycrystalline diamond bonded to a backing layer of less hard material, such as cemented tungsten carbide. Since the backing layer is of less hard material than the facing layer, the two-layer arrangement of the cutting element provides a degree of self-sharpening since, in use, the less hard backing layer wears away more easily than the harder cutting layer.
Such preform cutting elements are often in the form of a circular tablet of substantially constant thickness or are derived from such tablets. For example, the elements may be sectors or segments of such circular tablet. Other polycrystalline diamond preforms comprise a unitary body or layer of polycrystalline diamond formed without a backing layer, and such elements may be thermally stable so that they may, for example, be incorporated in a matrix body bit during formation of the matrix.
In order that the entire surface of the bottom of the hole being drilled is acted on by the cutting elements, the elements are located at different distances from the central axis of rotation of the drill bit. However, cutting elements further from the axis of rotation, and nearer the gauge region, move more rapidly relative to the formation than elements nearer the axis of rotation and the annular area of formation swept by each such cutting element is greater. As a result, cutting elements nearer the gauge region tend to wear more rapidly than elements near the axis of rotation. In order to combat this it is often the practice to position more cutting elements nearer the gauge region. Where preform cutting elements are used it is usual for such cutting elements to be used all over the leading face of the bit, although bits have been manufactured where the preform cutting elements are supplemented by natural diamonds embedded in the leading face of the bit, particularly in the region around the axis of rotation where it may be difficult to fit sufficient preform cutting elements due to limitations of space. The disadvantage of such arrangement is that natural diamonds are not self-sharpening so that, as the drill bit wears in prolonged use, the diamonds may become less effective than the preform cutting elements in certain types of formation.
A drill bit having preform cutting elements over substantially the whole of the leading face is expensive to manufacture due to the comparatively high cost of the preform cutting elements themselves. The present invention is based on the realisation that, since cutting elements nearer the axis of rotation are less subject to wear than cutting elements further from the axis, as described above, such cutting elements may be replaced by a form of cutting element which is cheaper to manufacture than preform cutting elements, provided the cheaper elements still provide a degree of self-sharpening and also provide sufficient wear resistance for their location on the drill bit.